The world's largest animal rights group, People for the Ethical Treatment of Animals, has been criticised by the Advertising Standards Authority for misrepresenting the science behind animal experiments and unfairly denigrating scientists who use animals in their work.

The ASA upheld five complaints by a pro-vivisection group, the Research Defence Society, about a leaflet issued by Peta, which featured "facts" on animal experiments and invited readers to fill in a questionnaire and donate money to the organisation.

The RDS challenged claims that "nearly 3 million sensitive animals - monkeys, rabbits, mice and others - are killed in the UK each year in painful experiments" and that "animal experiments are crude and unreliable".

The ASA noted that laboratory animals might suffer from stress during experiments and that, while Peta had demonstrated that nearly 3 million animals had died last year in experiments, "they had not submitted evidence to demonstrate that nearly 3 million animals died as a result of painful experiments". It instructed Peta not to repeat the claims and now requires Peta to rewrite its leaflets.

On the issue of science, the ASA said that although Peta had shown evidence of a difference between human and animal physiologies, "the implication that physiological differences rendered the results of animal experiments crude or inapplicable to humans was misleading".

Simon Festing, director of the RDS, said: "This ruling demonstrates how animal rights activists attempt to raise funds through deceiving the public about the medical benefits of animal research. With a budget of over $28m [about £16m], we would have thought Peta could afford to check their facts from time to time."

Yvonne Taylor, of Peta, said that the ASA had been provided with masses of evidence to support its case. "Sadly, the ASA have chosen to adopt an old-fashioned attitude and chosen to rule in favour of RDS. It's not really surprising. There's a huge resistance to changing long-established conditions and we are talking about massive financial interests at stake. The pharmaceutical industry is the world's most profitable industry."

The ASA also concluded that Peta's description of the motivations of scientists was unfair. Its claim that animal research is a "gravy train fuelled by millions of pounds from taxes paid by people like you" was unlikely to be seen as an expression of opinion, according to the ASA, and could be misinterpreted as a claim that those carrying out animal experiments made large profits.

In December last year the ASA also upheld several complaints against a different anti-vivisection group, Europeans for Medical Progress. It ruled that a leaflet sent out by the group, which claimed that animal experiments had held back the treatment of childhood leukaemia and the search for cures for other cancers, was inaccurate and misleading about the science behind animal research.